Power conversion devices for converting a DC voltage into an AC voltage with medium- and high-frequency resonant inverters using current resonance are used for applications such as railcars or solar power generation (see Patent Literature 1). A current resonant inverter creates a moment at which a current is zero using resonance of an LC circuit within a switching circuit, and turns off a power semiconductor switch element at the timing, thereby reducing switching loss. The current resonance causes turn-off loss of the power semiconductor to be substantially zero, which advantageously provides a power conversion device with low losses.
When a switch is activated at a frequency different from a resonance frequency of the inverter circuit, the resonant inverter may have a risk of breaking a switch element because an off-resonance occurs and an overcurrent passes through the switch. For example, Patent Literature 2 discloses a control method for preventing a switch element from breaking due to an off-resonance in a series resonant converter. Patent Literature 2 discloses a half-bridge current resonant converter in which a current of a secondary rectifier diode, which does not flow during a steady operation (at a normal frequency), is detected to determine an off-resonance.
Patent Literature 3 discloses a method for controlling an induction-heating power source, in which when a switching loss at turn-off exceeds an allowable setpoint in a full-bridge current resonant inverter, operation of the inverter is stopped. Patent Literature 4 discloses a control circuit for a switching power source, in which, when an off-resonance is detected from a detected value of an output current of a complex current resonant converter, a switch element is turned off. Patent Literature 4 also discloses that a signal for turning a switch element on is blocked while a current flowing through an impedance element is detected with both of two switch elements turned off.